1. Field Of the Invention
The present invention is directed to an apparatus for generating acoustic pressure pulses in an acoustic propagation medium of the type having a membrane that adjoins the acoustic propagation medium and is driven in an impact-producing manner.
2. Description Of the Prior Art
An electromagnetic embodiment of a pressure pulse generator of the above general type is disclosed by German OS 37 42 500. The pressure pulse source employed in this pressure pulse generator is composed of an electrically conductive membrane and a flat coil adjacent thereto. The flat coil is connected to a voltage supply that contains a capacitor charged to several kV. The capacitor is caused to discharge suddenly across the coil, and the discharge current flowing therein builds up a magnetic field extremely rapidly. This field in turn simultaneously induces a current opposite that of the coil in the membrane which, consequently, generates an opposing magnetic field under whose influence the membrane is moved suddenly away from the coil. The resulting pressure pulse, for example, can be employed for the non-invasive disintegration of calculi situated in the body of a patient or can be employed for the non-invasive treatment of pathological tissue.
In order to achieve a high efficiency, i.e. an optimally complete conversion of the input electrical energy into pressure pulse energy, it is necessary to have the membrane lie flat against the coil. It must also be guaranteed that the membrane returns into its initial position after the generation of a pressure pulse before the next pressure pulse is generated. This, for example, can ensue, as disclosed in German OS 37 42 500, by evacuating the space between membrane and the flat coil during manufacture of the pressure pulse source, and hermetically sealing this space by suitable means. In the medical application of standard pressure pulse systems, the acoustic propagation medium such as, for example, water is usually circulated for cooling and for eliminating air bubbles that disturb the propagation of the shock waves. When a pressure pulse generator is positioned above a water supply reservoir, an under-pressure in comparison to the atmosphere occurs in the acoustic propagation medium corresponding to the height difference between the pressure pulse source and the supply reservoir. This under-pressure leads to tensile forces that pull the membrane away from the coil under certain circumstances, even given evacuation of the space between the membrane and the coil. If the space between the membrane and the coil is not absolutely tight relative to the surrounding atmosphere, this can lead to the formation of an air pillow in this space. If a larger air pillow forms in this way over an adequately long time span, then the air also remains enclosed when the pressure pulse generator is again located under the supply reservoir in the meantime, i.e. when the normal pressure again prevails in the acoustic propagation medium. Due to the loss in electromagnetic energy occurring as a consequence of the air pillow, the functioning of the pressure pulse source is thus no longer assured. Further, given a longer-lasting influence of tensile forces, a plastic material deformation of the membrane can occur, this destroying the reproducibility of the pressure pulses that is extremely important for the application. Moreover, the evacuation and hermetic sealing of the intervening space involve considerable technological outlay. The occurrence of an under-pressure in the acoustic propagation medium compared to the intervening space can be prevented according to German OS 41 33 327 by maintaining the closed space wherein the acoustic propagation medium is located at a static pressure that is elevated in comparison to the ambient pressure. In this case, however, the cooling of the acoustic propagation medium and the elimination of air bubbles that occur during the application must be resolved in some other way, which may require increased technological outlay under certain circumstances.